An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
There is a continuing need for efficient, stable, robust formulations for broadband light-emitting OLED devices. By broadband light, it is meant that the OLED emits sufficient light throughout the spectrum so that such light can be used in conjunction with filters to produce a full color display. In particular, there is a need for white light-emitting OLEDs where there is substantial emission in the red, green, and blue portions of the spectrum, wherein a white-emitting electroluminescent (EL) layer can be used to form a multicolor device. Full color OLED devices are well known in the art. Typical full color OLED devices are constructed of three different color pixels that are red, green, and blue in color. Such an arrangement is known as an RGB design and the individual color pixels are deposited by the use of a precision shadow mask. An example of an RGB design is disclosed in U.S. Pat. No. 6,281,634. Another method is to use a white light-emitting device in conjunction with R, G, B color filters. A white-emitting electroluminescent (EL) layer can be used to form a multicolor device. Each pixel is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixelated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340. Other examples of white-light-emitting OLED devices are disclosed in U.S. Pat. No. 5,683,823, JP 07-142169, and U.S. Pat. No. 5,405,709.
Full color organic electroluminescent (EL) devices have also recently been described that are constructed of four different color pixels that are red, green, blue, and white. Commonly assigned U.S. patent application Ser. No. 10/812,787 filed Mar. 29, 2004 by Michael E. Miller, et al., entitled “A Color OLED Display With Improved Power Efficiency”, the disclosures of which are herein incorporated by reference, describe such four-pixel design. Such an arrangement is known as an RGBW design. An example of an RGBW device is disclosed in commonly assigned U.S. Patent Application Publication 2002/0186214 A1. In an RGBW device, high efficiency white-emitting pixels are used to display a portion of the digital image content. This results in improved power consumption relative to an RGB constructed of similar OLED materials. For this purpose a white emitter with white color close to the target white point of the display (typically D65) is desirable. Numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed, and device performance has been improved. For the commercial success of OLEDs, further improvement in devices is needed. Among the key areas that need further improvement to achieve lower power consumption and longer lifetime are the efficiency and operating stability of the devices. It is also desirable to improve the color gamut obtainable from such devices.